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 Post subject: GRE Number TheoryPosted: Thu Sep 13, 2012 4:18 am

Joined: Sun May 30, 2010 3:15 am
Posts: 424
Which of the following integers is/are the product of exactly two different prime numbers?
21, 22, 23, 24, 25

21, 22

The correct answers are 21 and 22. Let's factor each possible answer.
Choice A: 21 = 3 × 7
Choice B: 22 = 2 × 11.
Choice C: 23 = prime number.
Choice D: 24 = 2 × 2 × 3 × 3.
Choice E: 25 = 5 × 5.
Only the first two choices (21 and 22) are the product of exactly two different prime numbers.

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23 is also the product of the two following different prime numbers:
23 and 1

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 Post subject: Re: GRE Number TheoryPosted: Thu Sep 13, 2012 4:18 am

Joined: Sun May 30, 2010 2:23 am
Posts: 498
Quote:
23 is also the product of the two following different prime numbers:
23 and 1
1 is NOT a prime number.

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